How do I make normals visible on both sides of a model?
You should simply add thickness to your walls, as they said in the comments, you don't have walls made only by a plane in reality, so the fastest way you can adjust your model is by using the solidify modifier just add as much thickness as you want and you should have something similar to the first picture; as you can see, this way you have normals on every side. In the second picture you can see where the thickness value is and all the other values you can play with in the solidify modifier.

Difference between solidfy and no solidify:



Solidify modifier: